Inkjet printing is commonly used for printing on paper or other types of print media as ink receivers and is generally a non-contact application of an ink to the print media. Typically, one of two types of ink jetting mechanisms are used and are categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CIJ). The first technology, “drop-on-demand” (DOD) ink jet printing, provides ink drops that impact upon a recording surface using a pressurization actuator, for example, a thermal, piezoelectric, or electrostatic actuator. One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TIJ).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink is perturbed using a drop forming mechanism such that the liquid jet breaks up into drops of ink in a predictable manner. One continuous printing technology uses thermal stimulation of the liquid jet with a heater to form drops that eventually become print drops and non-print drops. Printing occurs by selectively deflecting either the print drops or non-print drops and catching the non-print drops. Various approaches for selectively deflecting drops have been developed including electrostatic deflection, air deflection, and thermal deflection.
Additionally, there are typically two types of print media used with inkjet printing systems. The first type is commonly referred to as a continuous web while the second type is commonly referred to as a cut sheet(s). The continuous web of print media refers to a continuous strip of media, generally originating from a source roll. The continuous web of print media is moved relative to the inkjet printing system components via a web transport system, which typically include drive rollers, web guide rollers, and web tension sensors. Cut sheets refer to individual sheets of print media that are moved relative to the inkjet printing system components via rollers and drive wheels or via a conveyor belt system that is routed through the inkjet printing system.
Inkjet printing with aqueous inks on smooth receivers (such as glossy clay coated lithographic paper stock) and non-porous receivers suffers from a well known quality problem known as ink coalescence. Non-porous is defined as a receiver that does not absorb freshly jetted ink deposits quickly enough into the receiver before the ink can dry thus permitting the undesirable effect of ink spreading and coalescencing into puddles. Text characters will have thin and thick areas of ink; this can resemble beads on a string. What is jetted as uniform density large solid areas will instead show macroscopic patterns of thin and thick ink deposits; the result can be described as either mottle or graininess depending on the spatial frequency of the coalescence. Additionally, ink of different colors that is not rapidly absorbed into the receiver may mix when jetted into adjacent areas resulting in image bleed.
A half-tone process is a well known dithering technique to reduce imaging instabilities of ink or toner deposition processes by dot area modulation of high deposition spots and areas of little or no deposition. The amount of deposition of toner or ink within the high deposition spots may also vary to produce a wide range of toner coverage or ink image densities. Inkjet dots are produced on a production print by discrete drops of ink and are typically limited to one or three drop sizes and to resolutions below 1200 dots per inch (DPI). These limitations prevent the use of half-tone dots in a regular pattern at a resolution pleasing to the eye. Instead, an error diffusion dithering processes must be used. A problem with error diffusion dithering images is that the image appears grainy. The grainy appearance may increase for color images as the error diffusion patterns in each color separation are over laid and increased image grain may be observed with an increasing number of color separations.
Certain inkjet printers can be adapted to produce high quality inkjet images (such as for proofing) by the use of multiple passes of imaging with high addressable ink printheads (4800 DPI) and inks having lower pigment or dye concentrations to overcome the grainy effect. To prevent the ink from coalescing, the ink vehicle, which may be water or a solvent, must partially be removed between imaging passes resulting in a slow imaging process. Smooth receivers such as clay coated paper are preferred for high quality inkjet images requiring long dry times between passes or expensive porous pre-coats to absorb the ink vehicle. Therefore, there is a need for a method of producing half-tone inkjet images in a single pass without a grainy appearance.
U.S. Pat. No. 6,140,390A, U.S. Pat. No. 6,753,051B1, U.S. Pat. No. 7,335,407B2, U.S. Pat. No. 7,858,161B2, and U.S. Pat. No. 8,298,634B2, all incorporated herein by reference, include inkjet receiving layers comprising polymeric particles and other ingredients, which are coated onto receiver base stock via liquid based coating processes. Such receivers must be manufactured via large scale controlled and complex industrial operations. There is a need for a pre-coat process that can be accomplished in the actual inkjet printer. There is also a need for a pre-coat process that can be utilized on any type of receiver, without having to purchase a specialty coated stock as described in these references.
U.S. Patent Publications US20130162703(A1), US20130130172(A1), US20130127964(A1), US20130127149(A1), and US20130129393(A1), all incorporated herein by reference, describe dry polymeric particle pre-coating processes for inkjet printing, that do solve some of the problems in the previously described prior art. These US Publications teach combining clear toner and inkjet ink to facilitate de-inking. None of these references teach changing the micro-structure of the toner lay-down to prevent coalescence nor do they teach using micro-structure patterns to improve image quality.
A known problem with printing using half-tone patterns results when using an imaging process having more than three color separations and optionally black, also known has hi-fidelity color rendering (U.S. Pat. No. 5,155,599 and U.S. Pat. No. 5,745,120) is the generation of objectionable Moiré interference patterns known in the art as Moiré. As the number of color separations in the image increases, the choice of half-tone screen patterns becomes limited. For more than three color separations, a different line frequency may be required and not all color separations are printed at the optimal screen frequency. There is a need to be able to print a large number of color separations without being limited by Moiré.
Although satisfactory, there is a need to improve upon these disclosures. The present invention improves upon these disclosures by applying the polymeric particles in a half-tone pattern, the tone value of which is dependent on the density of the image to be inkjet printed in order to achieve optimal image quality.